On August 16, 2016, brush fires in California created multiple line faults on the 500 kV transmission system. During these events, the inverters at multiple bulk electric system (BES) solar sites ceased injecting current into the system, with the most significant event dropping 1200 MW of generation. In general, the solar sites did not trip, but rather, the inverter control systems either calculated an erroneous frequency or responded to low voltage and stopped gating the transistors. Once the faults cleared and the control systems sensed the recovery of frequency and voltage, they began re-injecting current after a time delay that varied from 10 cycles to 5 minutes.
The California event brought to the forefront two very important issues. The first was that a transmission fault could impact multiple solar sites, which in this event was a maximum of approximately 1200 MW. The second was the programming and the speed of the inverter control systems. Upon analysis of fault recorder traces, it was found that the control systems of inverters could sense a system disturbance and respond by ceasing injection before the transmission system could clear an EHV Zone 1 fault in three cycles. Further analysis determined the inverter control systems either ceased injection in response to near-instantaneous erroneous frequency measurements or were configured to cease injection if the voltage was outside of the range of 1.1 to 0.9 per-unit.
After the events in California, the NERC/WECC joint task force was assembled by the NERC Operating Committee (OC) to analyze this disturbance, determine the causes, and develop key findings and recommendations to ensure that occurrences such as this one are mitigated throughout the North American BPS. The report can be found here:
The Report was followed by a NERC Alert Loss of Solar Resources during Transmission Disturbances due to Inverter Settings asking Generator Owners of solar PV plants to evaluate their inverters to determine if they are susceptible to responding to erroneous frequency calculations or abnormal voltages. In addition, the Generation Owners have been asked if the susceptibility has been mitigated by implementing manufacturer recommendations.
The issue is much bigger than responding to this event. At the FERC Reliability Technical Conference in June, NERC spoke of broad concerns for the system, noting the rapid pace of change as the generation resource mix is moving from large synchronous generating stations to renewable generating stations that are geographically dispersed and remotely operated. In addition, the existing synchronous generation is becoming increasingly fueled by natural gas, the renewable generation technology is changing at a rapid pace, and both battery storage and distributed energy resources (DERs) are growing at a rapid pace.
The North American Generator Forum (NAGF) believes there has never been a more important time for Generator Owners and Generator Operators to become involved with industry committees and task forces. As different industry experts have noted, it is time to figure out the attributes that hold the system together and establish the needs, values and compensation for those attributes. It has been the position of the NAGF that our primary role is to communicate to the system planners, operators and the regulators the attributes of our equipment in the form of generator technical capabilities, generator technical constraints and generator economics. Working with the regulators, markets and system operators to define asynchronous and synchronous generation capabilities, constraints and economics will help not only generator owners and operators, but the industry in general, as we figure out how to reliably operate a changing system.
NERC is moving forward with the Operating and Planning Committee-driven Inverter-based Resources Performance Task Force (IRPTF), which I was asked to chair. The IRPTF will be more forward-looking with the goal to explore the performance characteristics of inverter-based resources (e.g., solar photovoltaic (PV) and wind power resources) directly connected to the bulk power system (BPS). Recommended performance characteristics will be developed along with other recommendations related to inverter-based resource performance, analysis, and modeling. In this effort, we will be reviewing the coordination of Essential Reliability Services – inertia, voltage support, frequency response, ramping, etc. The technical capabilities of synchronous generation have been understood for decades, and the knowledge of the technical capabilities of wind and solar is growing quickly, but we are also seeing rapid change in those technologies, with one OEM noting that 5-year old inverter technology is ’ancient technology’.
As we move forward with a changing system, I hope that we as owners and operators of generation will continue to increase our involvement as we all work together to figure out how to plan and operate the system of the future.
Allen D. Schriver, P.E.
Chief Operating Officer, North American Generator Forum